1. Field of the Invention
This invention relates to a communication unit, and particularly relates to a communication unit having an orthogonal demodulation circuit.
2. Description of the Related Art
Operation of a conventional communication unit having an orthogonal demodulation circuit is described with reference to FIG. 1.
A signal received by the antenna 101 is amplified by the reception amplifier 103, and subjected to orthogonal detection by an orthogonal detector 110. The orthogonal detector 110 comprises multipliers 106 and 107 and oscillator 109, then a signal from the reception amplifier 103 is inputted to the one input terminals of the respective multipliers 106 and 107. A local signal that is the output from the oscillator 109 is inputted to the other input terminal of the multiplier 106, and the signal having the phase shifted 90 degrees is inputted to the other terminal of the multiplier 107. The outputs from the multipliers 106 and 107 are inputted respectively to the amplifiers 111 and 112, and also to an error detection circuit 203. The outputs from the amplifiers 111 and 112 are demodulated by the demodulation circuit 202.
On the other hand, a signal outputted from the modulation circuit 119 is subjected to orthogonal modulation using the local signal outputted from the oscillator 118 in an orthogonal modulator 117, and sent out from the antenna 101 through a transmission amplifier 116 and switch circuit 201.
The output of the transmission amplifier 116 is inputted to the reception amplifier 103 through an attenuator 201a in the switch circuit 201 as a local loop back signal. The local loop back signal is subjected to orthogonal detection in the orthogonal detection circuit 110 and inputted to the error detection circuit 203 like the reception signal from the antenna 101.
In a communication unit having such structure, when a power is on, a direct current offset error and gain error of the reception signal and local loop back signal are detected by an error detector 203, and fed back to the reception amplifier 103 and transmission amplifier 116.
A communication unit provided with automatic adjusting function of the sort is disclosed, for example, in Japanese Patent Laid-Open No. Hei 6-268553.
However, the above-mentioned communication unit requires a circuit exclusively used for correction of DC offset component of the reception signal, the requirement results in a large circuit scale because the conventional communication unit monitors the output signal outputted from the orthogonal detector and feeds back the error information to the amplifier on a hardware basis.
It is difficult to detect a DC offset component of the same channel just before the reception timing in time-division multi access (TDMA) because the detection output of the reception signal is used for DC offset detection, and therefore DC offset correction in reception channel during calling is difficult.